1. Field of the Invention
The present invention relates to a carrier-supported catalyst used for the synthesis of unsaturated aldehydes and unsaturated carboxylic acids, more specifically a carrier-supported catalyst used for synthesizing acrolein and acrylic acid, or methacrolein and methacrylic acid through gas phase catalytic oxidation of propylene, isobutylene or tertiary butanol with molecular oxygen, and also to a process for preparing the same.
2. Description of the Related Art
A variety of methods have been proposed for the preparation of the catalysts used for the synthesis of unsaturated aldehydes and unsaturated carboxylic acids. Taking up the case where isobutylene or tertiary butanol is used as starting material, many proposals have been made, as in JP-A-57-130949, JP-A-59-31727 and JP-A-60-28824, regarding the catalysts used for the generation of methacrolein and methacrylic acids through catalytic oxidation of isobutylene or tertiary butanol in a high-temperature gas phase. In these proposals, however, little or no mention is made of carrier-supported catalysts, let alone a carrier-supported catalyst using glass fiber as carrier assistant.
It is known to incorporate inorganic fiber as a molding assistant in a catalyst used for molding. JP-B-2-36296 discloses a heteropoly-acid-based catalyst using whisker as molding assistant. It is stated in this publication that the above catalyst is unsuited for use as a carrier-supported catalyst as it is unable to produce a sufficient catalytic activity in use as a carrier-supported catalyst, and that the whisker used as molding assistant is preferably the one having an average diameter of not greater than 5 .mu.m. On the other hand, according to the process for preparing a carrier-supported catalyst for the synthesis of unsaturated aldehydes and unsaturated carboxylic acids using molybdenum and bismuth as essential active components disclosed in the present invention, it is possible to obtain, with good reproducibility, a carrier-supported catalyst having a satisfactory catalytic activity by using glass fiber having an average diameter in the range of more than 5 .mu.m and not more than 200 .mu.m as carrier assistant. It is evident that the above publication is essentially different from the present invention.
In the prepation of a catalyst suitable for the production of acrolein and acrylic acid, or methacrolein and methacrylic acid through gas phase catalytic oxidation of propylene, isobutylene or tertiary butanol with molecular oxygen by a fixed bed reactor, in view of the fact that this reaction is an exothermic reaction, it is desirable to shape the catalyst active substance while defining the thickness of the catalyst layer for avoiding the undesirable rise of temperature due to heat accumulation in the catalyst layer. Definition of the catalyst layer thickness is also conducive to suppression of the collateral reactions of the product. Thus, a carrier-supported catalyst having an active substance supported on a carrier with a defined catalyst layer thickness proves in many cases favorable in terms of selectivity of the desired product.
A typical conventional carrier-supported catalyst producing method comprises dispersing a catalyst active substance or a catalyst precursor in a solvent to form a homogeneous solution or slurry, immersing a carrier therein or blowing said solution or slurry to the carrier to have said substance deposited on the carrier, and subjecting it to a heat treatment. This method, however, often proves unable to produce a catalyst with a satisfactory activity because of the limitation on the amount of the active substance that can be supported on a carrier.
It is known that the amount of the active substance supportable on a carrier can be increased by adding a hydroxide such as silica sol or alumina sol or an inorganic salt such as barium sulfate as carrier assistant. However, incorporation of such an additive material in the catalyst active substance tends to block the pores playing an important role for the reaction, resulting in a reduced catalyst performance. It is also known to add as binder a material that can be removed by a heat treatment, for example an organic compound such as oxalic acid, starch, polyvinyl alcohol, etc., or an inorganic salt such as ammonium nitrate. This method, however, has the problem that the mechanical strength of the obtained carrier-supported catalyst may be markedly reduced after removal of the binder, making the produced catalyst fail to have a sufficient strength for use as an industrial catalyst.
JP-A-59-173140 discloses a carrier-supported catalyst preparation method featuring use of whisker having an average diameter of not greater than 1 .mu.m as a carrier aid. This method, however, is still unsatisfactory in respect of mechanical strength of the produced catalyst, and there is a possibility of causing release or degradation of the catalyst during transport or charging into a reactor. Further, whisker is costly as compared with the inorganic fibers having a relatively large diameter, such as glass fiber or asbestos, and is therefore economically disadvantageous for use in preparation of an industrial catalyst. Also, the above publication states that in case of using fibers having a relatively large diameter, such as glass fiber, as carrier aid, the spray nozzle may be blocked when a slurry containing a catalyst active substance and inorganic fiber is blown to the carrier by using a spray. However, the present inventors found that when a shower having a sufficiently large nozzle bore and fibers with good dispersibility in a liquid material are selected, it is possible to perform the carrier applying operation without inviting any problem even when using fibers having a relatively large diameter.
JP-A-56-44045 discloses a method for obtaining a surface-coated type catalyst by dispersing a catalyst active substance or a catalyst precursor and inorganic or organic fibers in a solvent to form a slurry, immersing an inactive base material in the slurry, and drying or firing the same. Also, in an example of this publication, there is shown a method for obtaining a surface-coated denitrated catalyst by using a cordierite honeycomb as inactive base material while using glass fiber of 13 .mu.m in diameter as inorganic fiber. In the surface-coated catalyst producing method according to the above publication, since the catalyst substance is deposited on an inactive base material by means of immersion, the catalyst substance coating rate per run of coating operation is very low. In the case of a denitrated catalyst such as described in the above publication, the produced catalyst can well stand practical use even with a relatively low catalyst substance coating rate. However, in the the case of a catalyst to be used for the synthesis of unsaturated aldehydes and unsaturated carboxylic acids, when the catalyst active substance coating rate is of the degree specified in the above publication, it is impossible to obtain a catalyst having a sufficiently high activity required in practical use. It is therefore evident that the above publication is essentially different from the present invention.
Generally, further improvements are required of the carrier-supported catalysts from the industrial standpoint, particularly in terms of practicality of the production process, activity of the obtained carrier-supported catalyst, selectivity of the desired product and mechanical strength of the catalyst.